1. Field of the Invention
This invention relates to semiconductor light emitting diodes (LEDs) and more particularly to LED packages with optical elements.
2. Description of the Related Art
Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED. The useful light is generally emitted in the direction of the LED's top surface.
Conventional LEDs cannot generate white light from their active layers. One way to produce white light from conventional LEDs is to combine different colors from different LEDs. For example, the light from red, green and blue LEDs, or blue and yellow LEDs can be combined to produce white light. In addition, different colors of light are often generated from different types of LEDs which can require complex fabrication to combine in one device. The resulting devices can also require complicated control electronics since the different diode types can require different control voltages. Long term wavelength and stability of these devices is also degraded by the different aging behavior of the different LEDs.
Light from a single blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye. [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc., which comprise blue LEDs surrounded by a yellow phosphor powder; see also U.S. Pat. No. 5,959,316 to Lowery, entitled Multiple Encapsulation of Phosphor-LED Devices.] The surrounding material downconverts the wavelength of some of the LED light, changing its color. For example, a nitride based blue emitting LED chip can be surrounded by a yellow phosphor. Some of the blue light will pass through the phosphor without being changed while the remaining light will be downconverted to yellow. The LED will emit both blue and yellow light, which combine to produce white light. Another example of LEDs using this approach includes U.S. Pat. No. 5,813,753 to Vriens et al.
LED packages typically have some type of encapsulant surrounding the LED chip to enhance light extraction from the chip and protect the chip and related contacts structure (e.g. wire bonds) from exposure to physical damage or environmental conditions which could lead to corrosion or degradation. Along with this encapsulant, an optical element such as a simple hemispherical lens is also desired to enhance light extraction from the package and possibly to provide some form of output light beam shaping (control over the angle-dependent emission properties of the lamp). For surface mount packages, which typically require high temperature (200-300° C.) solder reflow processing to attach the LED package to its final fixture, the possible materials typically include silicones and glasses. Silicone lenses are typically molded using injection molding processes, which can place limitations on the properties of the silicone that may be used. Glass lenses are typically formed using a melting process that can limit the possible geometries and add substantial piece part cost to the final lamp. Typical wire bonded LEDs cannot be encapsulated in molten glass because of the high melting temperature of glass.
Present surface-mount LED package technology typically utilizes either a separate glass lens or a molded silicone lens. These lenses represent separate piece parts that must be affixed to the part during package assembly. The parts also represent additional cost due to the required lens fabrication, and may be difficult to attach. The lenses can also be made of materials (e.g. silicone) with a different coefficient of thermal expansion (CTE) compared to the surrounding package components. This can result in the cracking or delaminating of the LED chip or package elements, both of which can reduce light extraction from the LED package. This difference in CTE can also result in damage to the LED chip, and in particular the wire bond can be broken or pulled from the LED resulting in failure of the LED package.